Exhaust equipment members for automobile engines, such as exhaust manifolds, turbocharger housings, catalyst cases, exhaust manifolds integral with turbocharger housings, exhaust manifolds integral with catalyst cases, exhaust outlets, etc. are required to have improved heat resistance such as oxidation resistance and thermal crack resistance as well as high durability and long life, because they are used in such severe conditions as repeatedly exposed to high-temperature exhaust gases from engines with direct exposure to sulfur oxides, nitrogen oxides, etc. in the exhaust gas. The exhaust equipment members have conventionally been formed by inexpensive, high-Si, ferritic spheroidal graphite cast iron containing about 4% by weight of Si, which has relatively good heat resistance as well as good castability and machinability among the cast irons.
Because of recent improvement of the performance and fuel efficiency of automobile engines, and tightened regulations of exhaust gas emission, the exhaust gases tend to have higher temperatures. Accordingly, exhaust equipment members sometimes become higher than 800° C., so that higher heat resistance such as oxidation resistance, thermal crack resistance, etc. is required for the exhaust equipment members. Various improvements of the high-temperature properties of spheroidal graphite cast irons have thus been investigated.
Although conventional high-Si, ferritic spheroidal graphite cast irons have excellent castability and machinability at low production costs, their heat resistance such as oxidation resistance and thermal crack resistance is limited, so that exhaust equipment members made thereof cannot be used at temperatures exceeding 800° C.
JP9-87796A discloses a heat-resistant spheroidal graphite cast iron having a composition comprising, on a weight basis, 2.7-3.2% of C, 4.4-5.0% of Si, 0.6% or less of Mn, 0.5-1.0% of Cr, 0.1-1.0% of Ni, 1.0% or less of Mo, and 0.1% or less of a spheroidizing agent, the balance being substantially Fe, and a matrix based on a ferrite phase. This heat-resistant spheroidal graphite cast iron exhibits high oxidation resistance and thermal crack resistance in an environment subjected to repeated thermal load between 150° C. and 800° C., because of a relatively large amount of Si and small amounts of Cr and Ni added, so that it is suitable for exhaust equipment members for automobile engines, such as turbocharger housings, exhaust manifolds, etc. However, because this heat-resistant spheroidal graphite cast iron does not contain W, it is not necessarily sufficient in oxidation resistance and thermal crack resistance, failing to exhibit a satisfactory thermal cracking life particularly when used for exhaust equipment members repeatedly subjected to heating and cooling from room temperature to temperatures exceeding 800° C.
JP2002-339033A discloses a ferritic spheroidal graphite cast iron with improved high-temperature properties, which has a composition comprising, on a weight basis, 3.1-4.0% of C, 3.6-4.6% of Si, 0.3-1.0% of Mo, 0.1-1.0% of V, 0.15-1.6% of Mn, and 0.02-0.10% of Mg, the balance being Fe and inevitable impurities. The addition of V and Mn to a Si- and Mo-based composition improves not only high-temperature strength, thermal deformation resistance and thermal fatigue resistance, but also tensile strength and yield strength from room temperature to a high-temperature region of about 800-900° C., thereby increasing a life until initial cracking occurs, and improving thermal fatigue resistance. This is because V provides high-melting-point, fine carbide particles precipitated substantially in eutectic cell grain boundaries, thereby increasing grain boundary potential and preventing the pearlite structure from being decomposed at high temperatures, and because Mn accelerates the precipitation of the pearlite structure, thereby improving tensile strength and yield strength. However, because this ferritic spheroidal graphite cast iron does not contain W, it is not necessarily sufficient in oxidation resistance and thermal crack resistance.
JP10-195587A discloses a spheroidal graphite cast iron having a composition comprising, on a weight basis, 2.7%-4.2% of C, 3.5%-5.2% of Si, 1.0% or less of Mn, 0.03% or less of S, 0.02-0.15% of at least one of Mg, Ca and rare earth elements (including at least 0.02% of Mg), and 0.03-0.20% of As, the balance being Fe and inevitable impurities, with brittleness suppressed at middle temperatures around 400° C. This spheroidal graphite cast iron has improved high-temperature strength because it further contains 1% or less by weight of at least one of Cr, Mo, W, Ti and V as a matrix-strengthening component, and it also has improved ductility because of carbide suppressed by containing 3% or less by weight of Ni or Cu, a graphitizing element. Although the mechanism of suppressing embrittlement at middle temperatures is not necessarily clear, Mg remaining after the spheroidization, which is expected to segregate to crystal grain boundaries to cause embrittlement at middle temperatures, is combined with As to prevent the embrittlement function of Mg, and As remaining after combination with Mg improves the bonding of crystal grains, thereby mitigating or suppressing brittleness at middle temperatures.
However, because the amounts of Cr. Mo, W, Ti and V are as small as 1% or less by weight in this spheroidal graphite cast iron, it is not necessarily sufficient in oxidation resistance and thermal crack resistance when used for exhaust equipment members repeatedly heated and cooled. Also, the inclusion of As deteriorates the oxidation resistance of the spheroidal graphite cast iron at 700° C. or higher. In addition, As is toxic and extremely harmful to humans and the environment even in a trace amount, necessitating a facility for preventing operators from being intoxicated from the melting step to the casting step, and needing intoxication-preventing measures in the repair and maintenance of the apparatus. Further, it poses environmental pollution problems in the recycling of products. Thus, the As-containing, spheroidal graphite cast iron is not practically usable.
The conventional high-Si, ferritic spheroidal graphite cast iron has as low a ferrite-austenite transformation temperature (AC1 transformation point) as about 800° C., at which the matrix structure changes from a ferrite/pearlite phase to an austenite phase. The austenite has a larger linear expansion coefficient than that of the ferrite. Accordingly, when part of an exhaust equipment member becomes about 800° C. or higher, higher than the AC1 transformation point, the matrix changes to an austenite phase and so drastically expands, resulting in strain due to the expansion ratio difference. Also, when the temperature of the exhaust equipment member is lowered by engine stop, etc., the exhaust equipment member passes through the austenite-ferrite transformation temperature (Ar1 transformation point), resulting in strain due to the expansion ratio difference. Thus, the exhaust equipment member formed by the high-Si, ferritic spheroidal graphite cast iron is largely deformed by expansion and contraction due to the phase transformation in a state where it is constrained by other members by bolt fastening, etc. Also, repeated passing of the AC1 transformation point and the Ar1 transformation point causes the precipitation of secondary graphite, resulting in irreversible expansion and thus large deformation.
In addition, the exhaust equipment member is exposed to high-temperature exhaust gases containing sulfur oxides, nitrogen oxides, etc. and oxygen in the air at high temperatures, etc. (hereinafter referred to as “oxidizing gases”), resulting in oxide layers formed on the surface. When the oxide layers are heated to temperatures near the AC1 transformation point or higher and cooled, deformation and internal strain are generated by the difference in thermal expansion between the oxide layers and the matrix, resulting in micro-cracks in the oxide layers. The oxidizing gases penetrating through the cracks oxidize the inside of the exhaust equipment member (internal oxidation), so that cracks further propagate. The oxidation and cracking of the exhaust equipment member at high temperatures are thus closely related, both having large influence on the heat resistance, durability, life, etc. of the exhaust equipment member. Although the high-Si, ferritic spheroidal graphite cast iron containing about 4% of Si has a higher AC1 transformation point and thus higher oxidation resistance than those of usual spheroidal graphite cast irons, it exhibits insufficient oxidation resistance and thermal crack resistance when heated to 800° C. (the AC1 transformation point) or higher, resulting in a short life.
Accordingly, presently used for exhaust equipment members operable at temperatures exceeding about 800° C. in place of the conventional high-Si, ferritic spheroidal graphite cast iron having limited heat resistance such as oxidation resistance, thermal crack resistance, etc., are austenitic spheroidal graphite cast iron such as FCDA-NiCr20 2 (NI-RESIST D2), FCDA-NiSiCr35 5 2 (NI-RESIST D5S) containing about 18-35% by weight of Ni, etc., ferritic cast stainless steel containing 18% or more by weight of Cr, and austenitic cast stainless steel containing 18% or more by weight of Cr and 8% or more by weight of Ni, which have higher heat resistance than that of the conventional high-Si, ferritic spheroidal graphite cast iron.
However, the austenitic spheroidal graphite cast iron and the cast stainless steel are expensive because they contain expensive Ni or Cr. Also, because the austenitic spheroidal graphite cast iron and the cast stainless steel have high melting points, they have low melt fluidity and poor castability, so that they are likely to suffer casting defects such as shrinkage cavities, misrun, etc., and low casting yields. Accordingly, to produce exhaust equipment members at high yields, high casting techniques and special production facilities are needed. In addition, because they have poor machinability due to coarse carbides of Cr, etc., added in large amounts, high machining techniques are needed. With such problems, exhaust equipment members formed by the austenitic spheroidal graphite cast iron or the cast stainless steel are inevitably extremely expensive.
The internal oxidation of gray cast iron (flake graphite cast iron) in a high-temperature, oxidizing atmosphere appears to occur by the decarburization of graphite and the formation of oxides in the matrix by oxidizing gases intruding along three-dimensionally connected flaky graphite, resultant gaps and cracks accelerating the intrusion of oxidizing gases. To suppress the internal oxidation, the following proposals have been made.
(1) Flaky graphite having continuity is spheroidized, made finer, and reduced in their area ratio, to isolate graphite particles from each other, thereby suppressing the intrusion of oxidizing gases.
(2) 4-5% of Si is added to turn the matrix structure to silicoferrite, thereby elevating the AC1 transformation point.
(3) Carbide-stabilizing elements such as Cr, Mn, Mo, V, etc. are added to solid-solution-strengthen the matrix, thereby stabilizing pearlite and cementite.
However, any flake graphite cast irons and spheroidal graphite cast irons obtained by making graphite particles spheroidal, which are proposed above, fail to satisfactorily suppress the internal oxidation and heat cracking of exhaust equipment members used in environments at about 800° C. or higher.
The spheroidal graphite cast irons per se are long-known materials, and those having various compositions to be used for other applications than the exhaust equipment members have been proposed. For instance, JP61-157655A discloses a cast alloy iron tool comprising 3.0-7.0% of C, 5.0% or less of Si, 3.0% or less of Mn, 0.5-40.0% of Ni, 0.5-20.0% of Cr, and one or more of 0.5-30.0% of Cu, 0.1-30.0% of Co, 0.1-10.0% of Mo, 0.1-10.0% of W, 0.05-5.0% of V, 0.01-3.0% of Nb, 0.01-3.0% of Zr and 0.01-3.0% of Ti, the balance being substantially Fe, having a graphite area ratio of 5.0% or more, and a precipitated carbide or carbonitride area ratio of 1.0% or more. The wear resistance of this cast alloy iron is mainly provided by hard Cr carbide or carbonitride particles crystallized during casting. However, because the Cr carbide lowers toughness and ductility, this cast alloy iron does not have toughness and ductility necessary for the exhaust equipment members. In addition, because hard carbide or carbonitride particles lower the machinability, the cast alloy iron has low machining efficiency, resulting in increased production costs and thus expensive exhaust equipment members. Further, because it contains as much Ni as 0.5-40.0%, the ferrite-based cast iron (ferritic cast iron) has low AC1 transformation point and oxidation resistance, failing to achieve sufficient durability and life when used in environments higher than 800° C. Accordingly, heat-resistant cast irons suitable for exhaust equipment members used in environments higher than 800° C. cannot be conceived of from the cast tool described in JP61-157655A.
JP11-71628A discloses a composite roll with excellent thermal shock resistance comprising an outer ring made of tungsten carbide-based cemented carbide, and an inner ring made of spheroidal graphite cast iron and bonded to the outer ring by casting, the inner ring having a composition comprising, on a weight basis, 3-4.5% of C, 1.5-4.5% of Si, 0.1-2% of Mn, 0.02-0.2% of Mg, and 0.1-5% of one or more of Mo, Cu, Cr, V, W, Sn and Sb, the balance being Fe and inevitable impurities, and a structure having core-structure spheroidal graphite particles dispersed in a matrix based on a mixed phase of a ferrite phase and any one of a pearlite phase, a bainite phase and a martensite phase, and each core-structure spheroidal graphite particle comprising a core formed during the casting, and a shell precipitated during the heat treatment. To obtain the mixed phase of this spheroidal graphite cast iron, an as-cast pearlite phase-based matrix is first formed, a heat treatment comprising repeated heating and cooling in a temperature range between 450° C. and a solid phase line is conducted to form the ferrite phase, and the matrix is then turned to the mixed phase based on the pearlite phase and the ferrite phase.
However, when the spheroidal graphite cast iron of JP 11-71628A is used for exhaust equipment members operable in environments higher than 800° C., the pearlite phase, the bainite phase and the martensite phase are decomposed to precipitate secondary graphite, failing to exhibit enough durability by irreversible expansion. Among Mo, Cu, Cr, V, W, Sn and Sb, V deteriorates the oxidation resistance at temperatures exceeding 800° C., and Sn and Sb form abnormal flaky graphite in eutectic cell boundaries and cementite in the matrix when used in excess amounts, resulting in decrease in toughness and ductility, particularly decrease in room-temperature elongation. Accordingly, unless the alloying elements and their amounts are properly selected from Mo, Cu, Cr, V, W, Sn and Sb, it would not exhibit sufficient AC1 transformation point, oxidation resistance, thermal crack resistance, toughness and ductility as a material for exhaust equipment members used in environments higher than 800° C. Accordingly, heat-resistant cast irons suitable for exhaust equipment members used in environments higher than 800° C. cannot be conceived of from the composite roll described in JP 11-71628A.